1. The Field of The Invention
The present invention relates to a system and process for determining constants utilized in a process control system.
2. State of The Art
Process control systems are often utilized to control an industrial process. The process control systems generally include actuators such as motorized valves, motorized potentiometers, hydraulic valves, pneumatic valves, motor controlled lever arms, and the like. These actuators control such things as the flow of liquid in the process or the amount of electricity flowing to certain components of an electrical system. In computerized process control systems, computers are utilized to control the process control actuators. Generally, process control actuators have non-linear characteristics. Some of the typical non-linear characteristics associated with actuators are non-linear gain, backlash, stiction. In order for the process control system to function correctly and efficiently, the operating characteristics of the actuators and the controlled process must be known and available to the computerized controllers. The characteristics are normally called "tuning constants" or "tuning parameters". The process of finding the best value of the tuning constants of the actuators and the process, and making the values available to the controller is called "tuning" of the control loop.
Previously, the tuning constants have often been obtained by a method known as a manual "bump test". In the manual bump test, a certain known control signal is applied to the actuator, and the resulting change in the output of the controlled process is measured. Then the magnitude of the control input to the actuator is changed (bumped) and the corresponding change in the output of the controlled process is measured. These procedures are then repeated for a wide range of control inputs to the actuator, and then from the input values and the corresponding output changes and by using certain mathematical formula an operator manually computes the tuning constants for the actuator and the process. This manual bump test process is tedious and time consuming and is also subject to many human errors. Also, it causes a noticeable, sometimes objectionable, disturbance to the process which is being controlled by the actuator. For these reasons, the manual bump test is usually done as infrequently as possible.
However, the characteristics of actuators and the process are constantly changing during operation as a result of wear in parts, condition of lubrication, temperature, aging, and so forth. These changes in actuator and process characteristics very often necessitate periodic recalculation (retuning) of the control parameters to keep the loops in their best condition. However, as pointed out above, there are undesirable characteristics associated with the manual bump test and therefore one must choose between the undesirable results of utilizing incorrect tuning constants versus the undesirable effects of manually doing a bump test.
In contrast to the manual method of determining tuning constants, the present method employs a computerized, automatic system. Thus, tuning constants can be determined without the introduction of human error, and with little or no adverse effect upon the controlled process. Furthermore, with the present system and process, tuning constants can be determined far more quickly than by means of the manual method. Consequently, it is possible with the present system to determine tuning constants frequently and automatically so that tuning can be scheduled for periodic execution thereby permitting the utilization of far more accurate tuning constants in the operation and control of the process.